With the recent spread of application of ceramics to mechanical structural parts, such as automobile engines, it has been demanded to overcome brittleness inherent to ceramics to develop highly tough and reliable ceramic materials.
Silicon nitride (Si.sub.3 N.sub.4)-based ceramics mainly comprising Si.sub.3 N.sub.4 are excellent in strength at room temperature, oxidation resistance, abrasion resistance, thermal impact resistance, corrosion resistance, etc., and have been put into practical use as cutting tools or automobile parts.
In the latest developments of high efficiency engines, use of Si.sub.3 N.sub.4 as a high-temperature resistant material has been attempted. There has been a need, to this effect, to achieve further improvements in toughness and high-temperature strength at 1200.degree. C. or higher.
It is known that toughness of Si.sub.3 N.sub.4 can be improved by dispersing reinforcing SiC whiskers in a matrix of Si.sub.3 N.sub.4 to obtain a composite material as proposed in JP-A-62-265173 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, whisker-reinforced composite materials produced by conventional processes do not show an appreciable improvement in toughness which would have been brought about by the reinforcement. That is, high-temperature strength of the state-of-the-art whisker-reinforced composite materials is higher than that of an Si.sub.3 N.sub.4 single phase but is still lower than their room-temperature strength.
An improvement in toughness brought about by reinforcing with whiskers is believed attributed to deflection of cracks to the direction along whiskers, a pull-out effect of whiskers, a bridging effect of whiskers, and the like. However, where whiskers are merely dispersed in a matrix, both are reacted with each other on their interface to be firmly bonded to each other. In such a state, no substantial pull-out effect is produced when stress is imposed, only resulting in cutting of the whiskers, and no improvement in strength and toughness can be obtained.
In order to endow a whisker-reinforced ceramic composite material with further improved toughness, it is considered effective to control an interfacial reaction between a ceramic matrix and a whisker reinforcement. Coating of whiskers with a different substance has been studied in this direction. For example, JP-A-1-133981 discloses coating of SiC whiskers with partially stabilized ZrO.sub.2 by a CVD method.
As above exemplified, coating of a metal oxide on whiskers has conventionally been carried out by gas phase synthetic processes, e.g., a CVD method and a sputtering method. According to these processes, however, since deposition of a gaseous phase metal oxide shows directionality, it is difficult to coat the back side of whiskers. In other words, it has been very difficult to form a uniform coating on the entire surface of whiskers. Thus, ceramic composite materials reinforced with conventional metal oxide-coated whiskers do not undergo sufficient improvement on toughness and still lack reliability.
In addition, coating by gas phase synthetic processes is unsuitable for mass production and uneconomical because it requires a specially designed film-forming device and complicated and strictly controlled conditions or operations and also because the amount of whiskers which can be coated at a time is limited by the size of the film-forming device.
For example, it has been proposed to coat BN or C on the surface of SiC whiskers to provide a reaction inhibitory layer on the interface between the SiC whiskers and an Si.sub.3 N.sub.4 matrix as disclosed in JP-A-1-179762. According to this proposal, it is expected that the reaction between SiC whiskers and the Si.sub.3 N.sub.4 matrix can be inhibited by the carbide, nitride or carbon layer provided on the whiskers to reduce interfacial bonding strength, thereby obtaining a marked pull-out effect of the whiskers. Notwithstanding, it turned out that sufficient improvements in toughness and strength which would have resulted cannot be necessarily obtained.